Exhaust gas treatment devices have hitherto been used for the purpose of detoxifying the NOx, PM, and the like contained in exhaust gases discharged from vehicle engines or from internal combustion engines of construction equipment and the like. Such exhaust gas treatment devices have a configuration where a catalyst support made of a ceramic or the like is housed in a catalyst casing having the structure of a metallic shell or the like. Usually, this configuration includes a holding material interposed between the catalyst support and the catalyst casing. The holding material has both the purpose of fixing the catalyst support to prevent any damage due to vibrations and the purpose of serving as an exhaust gas sealing material, and alumina-based fibrous masses are being mainly used (see, e.g., Patent Document 1).
The exhaust control was recently made stricter, and the exhaust gas cleaners have come to be required to have higher functions accordingly. Then the alumina-based fibrous masses as holding materials are coming to be required to retain a retention ability (areal pressure) under severer use conditions.
For example, Patent Document 2 discloses a process for producing an alumina-based fibrous mass by conducting, a spinning step, a chopping step, a mat formation step, and then a burning step. Namely, the production process described in Patent Document 2 is that the continuous long-fiber precursor obtained in the spinning step is cut with a cutter or the like in a state of being unburned and flexible, to thereby obtain a short-fiber precursor. It is stated that such alumina short fibers obtained through these steps have suffered little cut-surface chipping and have few microcracks as compared with alumina short fibers obtained through cutting performed after a burning treatment and hence have high mechanical strength and reduced unevenness and that, because of these, an alumina-based fibrous mass which has a sufficiently high initial areal pressure and is less apt to deteriorate with the lapse of time can be obtained.
Meanwhile, there is a method being investigated in which the volume of pores that are present in alumina-based fibers and have a diameter within a specific range is reduced in order to improve the mechanical strength of the alumina-based fibers (see Patent Document 3). However, the alumina-based fibrous masses disclosed therein are limited to ones which have chemical compositions including 90% or 95% Al2O3. The alumina-based fibrous mass described in Patent Document 3 is insufficient in areal pressure for use, for example, as a holding material for the exhaust gas cleaners of motor vehicles where intense vibrations occur. There is a desire for an alumina-based fibrous mass having a higher areal pressure.